Background: Adverse effects of thiopurine drugs occur in 15-28% of patients and the majority is not explained by thiopurine-S-methyltransferase deficiency. Furthermore, approximately 9% of patients with inflammatory bowel disease are resistant to azathioprine therapy. Recently, the small guanosine triphosphatase, Rac1, was identified as an important molecular target of 6-thioguanine triphosphate, one of the active metabolite of thiopurines such as azathioprine. To date, no functional genetic polymorphism of the human Rac1 gene had been reported.
Objectives: Evidence for functional genetic polymorphisms of the human Rac1 gene and to investigate their relative contribution to the development of toxicity induced by azathioprine treatment in patients with inflammatory bowel disease.
Methods: We first screened for polymorphisms in the Rac1 gene in genomic DNA samples from 92 unrelated Caucasian individuals. The functional consequences of identified polymorphisms were assessed in vitro using transient transfection assays in Jurkat and A549 cell lines. The relationship between polymorphisms of Rac1 and thiopurine response or hematotoxicity was studied in 128 patients under thiopurine treatment.
Results: Three single nucleotide polymorphism and one variable number tandem repeat were identified in the promoter region of Rac1 gene. Interestingly, in Jurkat T cells, the c.-289G>C substitution and c.-283_-297[3] variable number tandem repeat displayed a significantly increased promoter activity (P<0.01) of 150 and 300%, respectively, compared with that of the wild-type sequence. Patients with thiopurine-S-methyltransferase mutations presented a significantly increased probability of developing hematotoxicity (odds ratio=5.68, 95% confidence interval=1.45-22.23, P=0.00625). Moreover, among the 75 patients who did not develop hematotoxicity, there was a marginally overrepresentation of functional genetic polymorphisms of Rac1 (odds ratio=0.18, 95% confidence interval=0.02-1.49, P=0.079).
Conclusion: This study constitutes the first report of a functional genetic polymorphism that could affect Rac1 expression and thus modulate the risk of adverse drug reaction in patients under thiopurine treatment. A larger scale (case-control) study should enable us to confirm or cancel these preliminary results.